Random Lasing at Localization Transition in a Colloidal Suspension (TiO2@Silica)

Anderson localization of light and random lasing in this critical regime is an open research frontier, which besides being a basic research topic could also lead to important applications. This article investigates the random laser action at the localization transition in a strongly disordered scattering medium composed of a colloidal suspension of core-shell nanoparticles (TiO@Silica) in ethanol solution of Rhodamine 6G. The classical superfluorescence band of the random laser was measured separately by collecting the emission at the back of the samples, showing a linear dependence with pumping fluence without gain depletion. However, frontal collection showed saturation of the absorption and emission. Narrow peaks of approximately equal intensity are observed on top of the classical superfluorescence band, indicating suppression of the interaction between the peaks modes. The linewidth of these peaks is lower than that of the passive modes of the scattering medium. A method called fraction of absorbed pumping allowed us to infer that this peak's mode (localized modes) is confined to a shallow region near the input-pumping border.
We gratefully acknowledge financial support from FACEPE, FAPESP, FAPESQ, and CNPq (Brazil). E.J.-V. acknowledges FACEPE (grant 0116-1.06/13, APQ-0071-1.06/14) and FAPESQ. I.F.S. thanks the CAPES (Brazil) for the fellowships of masters. N.U.W. acknowledges CNPq (grants 402241 2012_6 and 307989 2013-5) and FAPESP (grant 18162-4 2012). C.L. acknowledges Spanish MINECO (grant MAT2015-68075-R (SIFE)) and the Comunidad de Madrid (grant S2013/MIT-2740 (PHAMA_2.0)).